In high-speed optical communications networks, optical signals received through an optical fiber link suffer distortions due to, among other things, chromatic dispersion (CD) and polarization mode dispersion (PMD). In a direct detection receiver, the inbound optical signal is made incident on a photodetector, the output of which is proportional to the square of the optical intensity. Because phase information is lost in a direct detection receiver, recovery of data from the received optical signal depends on analysing the amplitude envelope of the of the photodetector output. An emerging technique for accomplishing this makes use of digital sequence estimation techniques such as Maximum Likelihood Sequence Estimation (MLSE), Maximum a posteriori (MAP) estimation and Turbo-decoding.
A limitation of all such systems is that the symbol timing within the received optical signal must be accurately obtained before the sequence estimator can function successfully. This symbol timing is typically obtained using known clock recovery techniques. However, traditional clock recovery techniques suffer a limitation in that they are based on Phase-Lock Loops (PLLs), which have a very limited tolerance to inter-symbol interference (ISI) produced by CD and PMD. This means that the signal reach and link bandwidth are limited by the clock-recovery methods implemented in the receiver, which is undesirable.
A further limitation of PLL-based clock recovery methods is that signal noise is normally assumed to be linear, and exhibit a Gaussian spectral distribution. Amplified Spontaneous Emission (ASE) is a typical example of such linear, Gaussian noise, and is often used for modelling channel noise for design and testing of receiver systems. However, in a direct (square-law) detection receiver, channel noise in the photodetector output tends to be highly non-linear, and frequently non-Gaussian. This channel noise characteristic is outside of the design parameters of conventional PLL-based clock recovery techniques.
In order to address this issue, Godard clock recovery and analog phase frequency detectors have been proposed. See, for example, D. N. Godard, “Passband Timing Recovery in an All-Digital Modem Receiver,” IEEE Trans. Comm., Vol. COM-26, No. 5, May 1978. However, Godard clock recovery techniques suffer a limitation in that they require T/2 spaced sampling of the received signal. Thus, the received signal must be digitally sampled at a sample rate double that of the signal line rate. A lower line rates, this requirement can be accommodated. However, at line rates above about 20 Gbaud, the heat generated by the required high speed digital circuits (Analog-to-Digital converter, digital signal processor etc.) increases costs and degrades system performance and reliability. Even when these issues are addressed, Godard clock recovery techniques still offer only a limited range of acquisition with respect to optical channel conditions (such as ISI). Analog Phase Frequency detectors also offer only a limited acquisition range.
High speed digital clock recovery systems capable of blind (that is, without knowledge of the data modulated in the signal) clock recovery are known, for example, from Applicant's co-pending U.S. patent application Ser. Nos. 11/315,342 and 11/315,345, both filed Dec. 23, 2005. However, these systems rely on coherent detection, which means that the channel noise remains linear and Gaussian.
Robust techniques capable of achieving a lock condition in the presence of non-linear Gaussian channel noise remain highly desirable.